1. Field of the Invention
This invention pertains to the field of flow-control devices. More particularly, it relates to motor-driven control valves and corresponding control circuit for use with them that combines the unique concepts of limiting the current flow to the valve drive motor to overcome stiction, providing for instantaneous braking, and utilizing controlled rotation of the motor for precise valve positioning. Damage prevention is obtained by elimination of over-rotation thereof through utilization of a normally open limit switch, limiting the motor drive gross movement of the valve stem.
2. Description of the Prior Art
Many processes involve the flow of liquids and gasses in tubes and pipes and require controlled regulation. Needle valves, regulating valves and shut-off valves are examples of controlled regulation used to control the flow of these materials. In multiple valves or remote location applications, the industry adopted a convention whereby motors drive the valves from fully open to fully closed positions and to desired positions therebetween. The motors used in the industry include electric, stepper and pulse motors and the means to control them vary from simple on-off switches to feedback mechanisms coupled to computer circuitry.
An electric-drive motor, usually a direct current driven motor of the reversible type, is connected to the valve stem that protrudes from the valve body. Control means are attached to the motor to provide positive or negative direct current to drive it and the valve stem in one direction or another. The motor is usually small, having very high speed output, usually in the 4,000 to 6,000 rpm range. The motor is connected to the valve stem through a transmission that gears the high incoming rpm down to a very low range of 10 to 20 rpm. Certain problems have developed in this field and have not yet been solved so that the full utilization of motor control has not yet occurred.
For instance, one problem concerns the application of motor control to valve closure. Through repeated opening and closing of the valve, the valve seat wears, thus making the valve element that closes against the valve seat travel further into the valve housing. Since virtually all valve elements advance toward and away from the valve seat through screw threads, the wearing of the valve seat requires the valve element and stem extending therefrom to close against the valve seat at progressively different angular positions. This means that the valve cannot be predicted to close at any particular angular position of the stem because the slightest wear on the seat will prevent the angular position from insuring that the valve is closed. When this occurs the valve will leak.
In the prior art, valves are set to be closed by ordering the drive motor to turn the valve stem until it stops turning, i.e., has forced the valve element fully against the valve seat. To little motor power will not ensure a fully closed valve and too much motor power may cause the valve element to mash hard against the valve seat, causing increased wear in the valve or damage to the transmission gears and other components. To avoid these situations, the prior art has established the practice of sizing the drive motor to stall at the maximum friction load needed to just close the valve. In other words, the motor will just close the valve and remain in a stall condition to hold the valve shut. This convention wastes electrical power during extensive valve-closure periods, and causes wear on the motor and drive gears in the form of vibration, called "chatter". Furthermore, should a power failure occur during this valve-closure hiatus, the drive motor would cease its electrical stall and possibly allow the closed valve to drift open and allow undesired backflow.
Also, there is the problem known as "stiction". This term comes about because of frictional buildup in the valve. While the valve stem is in motion, there is generally constant friction encountered in the valve and the load on the drive motor remains relatively uniform. That is to say, there is no buildup of forces in the valve itself and the movement from full-open to nearly full-closed position may be handled by the drive motor without difficulty. However, when the valve reaches the fully-closed position, a sudden increase in frictional load occurs in the valve stem because of tightness achieved between the valve parts as well as some friction buildup caused by flow interruption in the line. To open a fully-closed valve, therefore, requires the drive motor to initially overcome this rather large frictional force or "stiction". Once the valve is cracked open by the drive motor, the stem friction drops to the relatively low value throughout the remainder of valve travel. With the prior art drive motor at stall condition, holding the valve closed, there is not enough additional power during reverse operation to overcome this stiction and the valve often remains closed until movement is started by hand.
In my previously issued U.S. Pat. No. 5,137,257, I have disclosed and claimed a means freely rotatable with the motor drive shaft for providing a controlled amount of overturning of the motor-drive shaft following full closure of the valve element against the valve seats and simultaneously storing a portion of the drive energy expended in the overturning to bias the valve in its closed position, and to discharge the stored energy to aid in opening the valve upon reverse turning of the drive motor and motor-drive shaft.
However, further problems have been determined to exist in the day-to-day operation of such a valve. Because of the high rotation speed of the motor and its sudden start-stop operation, the associated gears tend to wear and deteriorate over time, causing motor failure. Additionally, when electrical energy, in the form of DC current in the amount of 12 or 24 volts, is applied to the electric drive motor, there is a surge of current that passes into the motor and through the commutator that eventually subsides to an amount of current proportionate to the load on the motor. During this initial surge, or "spike" as it is known in the trade, arcing occurs between the motor brushes and the commutator such that, over a period of time, the commutator shows signs of wear and erosion and eventually fails, thereby rendering the motor unusable. Because the motor is so small and further because it is made by high-speed production techniques, the cost of repairing the motor is significantly large compared with the initial price thereof. It is not uncommon for motors, such as those presently used in motor-driven valves of the type herein described, to have a useful life limited to about 2500 hours.
Another significant problem exists with respect to the motor continuing to turn after power has been terminated thereto. The transmission, including the numerous gears, cause a significant load to the motor. The motor spins at very high rpm to drive the transmission that, in turn, drives the needle valve to its various controlled positions. The high rpm of the motor develops a significant inertia. The inertia does not allow the motor to stop immediately upon command. The motor continues to rotate powered by the inertia. Thereby the valve is turned beyond the point at which the controller signals a cessation of motor drive and the resulting valve position is not where it should be. When this happens, the controller must order the motor to reverse direction and bring the valve into proper position. This extra movement of the motor results in more wear which is a factor in early motor replacement. Finally, a limit switch is often placed in series with the valve stem. This limit switch operates independent of the controller and it establishes the point beyond which the valve stem is not to be turned, so that valve damage is held to a minimum. In the prior art, the limit switch is generally a normally-closed single pole switch, carrying current therethrough. Such use of the limit switch requires it to shoulder the burden of handling electrical current throughout the valve travel. Such use reduces the work-life of the limit switch. Its failure to continually carry current to the motor results in positioning the valve at undesired settings and disrupts the process in which the valve is an important part.